Internal combustion engines such as automobile engines are recently strongly demanded to have improved fuel efficiency. Accordingly, research and development have been widely carried out to achieve size reduction, weight reduction, friction loss decrease, etc. For example, attempts have been made to use aluminum alloys for cylinders, and to coat piston rings with low-friction-coefficient, hard carbon called diamond like carbon (DLC).
Though some aluminum alloy cylinders have portions in direct sliding contact with piston rings, which have insert-molded cast iron liners or plating, both requirements of cooling performance and cost on cylinders have demanded to use for cylinders aluminum alloys having excellent wear resistance, for example, a hypereutectic Al—Si alloy having relatively hard primary Si crystals precipitated.
From the aspect of chemical stability and a low friction coefficient to aluminum alloys, attempts have been made to provide piston rings with hard carbon coatings. However, the practical use of hard carbon coatings have been largely hindered by their two essential properties that large residual stress exists because of their forming process, and that carbon bonds are chemically stable, resulting in low adhesion to substrates.
US 2012/0205875 A1 discloses a DLC-coated piston ring, the coating comprising a bonding layer, a metal-containing DLC layer, and a metal-free DLC layer in this order from inside to relax residual stress; a thickness ratio of the metal-free DLC layer to the metal-containing DLC layer being 0.7-1.5; and the coating being as thick as 5-40 μm. Extremely high internal stress of the metal-free DLC layer is offset by the metal-containing DLC layer having a similar thickness, thereby improving adhesion. It is described that the metal-free DLC layer preferably has hardness of 1700 HV0.02 to 2900 HV0.02, exhibiting excellent friction resistance and wear resistance in an experiment using a honed gray-cast-iron-made cylinder sleeve as a mating member.
However, when boundary lubrication occurs particularly in an early stage of operation of an engine with a hypereutectic Al—Si alloy as a mating member, detached Si particles act as abrasives, likely causing the local peeling of a coating due to increased local stress, failing to withstand a long period of use.